how will you model a cityscape at night?
I'd recommend modeling the fence, vegetation and other props in the foreground. 

Don't forget to get the grungy texturing right. 

And the perspective, which can be done easy with this PLUGIN, If you don't like it or have any technical problems there are many alternatives, there was a good one mentioned by Andrew Price, can't seem to remember what it's called now.

Then, render it with alpha background, by ticking Transparent under Film settings.

After, choose the right background and just plug it in Photoshop or Gimp.
Here's one of the top results in google image search: BACKGROUND

Then it's just post processing: Desaturation (Black and white) with high contrast. I'd further recommend going to blenderartists "work in progress" and let your peer artists guide you through the rest to get a more accurate result.